1. Field of the Invention
The present invention relates generally to delta-sigma modulators/converters, and more specifically, to a delta-sigma modulator having reduced signal level in the loop filter that is provided by an AC-coupled feedback path.
2. Background of the Invention
Delta-sigma modulators are in widespread use in analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), in which they provide very linear behavior and simple implementation due to the reduced number of bits used in the analog signal comparison. The delta-sigma modulator typically includes a loop filter and a quantizer connected in a feedback loop that combines the output of the quantizer with the input signal. The resulting operation provides noise shaping of the output of the quantizer, as determined by the loop filter characteristics. The path from the input of the modulator to the output of the quantizer (as also modified by the feedback), provides the signal transfer function (STF) and the loop around the quantizer loop filter provides the noise transfer function (NTF). Converters employing a delta-sigma modulator typically have a loop filter that optimizes the NTF to “shape” the “quantization noise” so that it is out of the signal band of interest, while providing a highly-linear response for the STF.
Originally, delta-sigma modulators for analog-to-digital converters (ADCs) employed single-bit feedback-only topologies, which have an advantage in that the STF is an all-pole response. However, the feedback topology modulator is disadvantageous in that each integrator output has an offset that compensates for the average DC value of the quantizer feedback provided to each integrator, raising the overall signal swing level required from each stage and leading to increased non-linearity. Also, the quantizer signal and quantization noise are provided to each integrator by the multiple feedback paths and are not attenuated as much in the feed-forward topology modulator, in which the quantizer output is introduced only at the input combiner. Thus, the feedback topology converter typically requires higher integrator time constants, and thus larger capacitors. Since the thermal noise floor of the loop filter is set by the input resistance, higher integrator time constants require larger capacitors, undesirably increasing required die area for implementation.
The higher signal levels present in the stages of a feedback-type modulator present several problems. First, in order to maintain the same level of harmonic distortion, the linearity of the loop filter stages must be higher than for a corresponding feed-forward design, since the signal levels through those stages are higher. In particular, the DC offset present at each stage of the feedback-type modulator contributes greatly to non-linearity. The capacitors used in the integrators must also be much more linear than for the feed-forward design for the same reason. Finally, for converters using multi-bit quantizer feedback, the capacitors must typically be larger in the feedback modulator design. Increasing the number of quantizer levels in the feed-forward design significantly reduces the integration gain/time constant requirement of the loop filter stages, whereas in the feedback topology, the decrease is not a significant.
Therefore, feed-forward topologies, and in particular, multi-bit feed-forward topologies, are almost always used in ADCs, due to the reduced noise and signal levels present in the integrator stages of the loop filter. The feed-forward topology has disadvantages in that there are zeros present in the STF. Out-of-band peaks in the response due to the presence of the zeros increase the converter noise floor by the aliasing of noise present at the peaks back into the signal pass-band.
Therefore, it would be desirable to provide a feedback topology delta-sigma modulator having reduced signal levels through the integrators, so that the noise performance advantages of the feedback topology can be had, without requiring large capacitors and highly-linear integrators and capacitors.